Melt processable copolymers with TFE and HFP are-well know under the name FEP. As perfluorinated thermoplasts, such copolymers have unique end-use properties like chemical resistance, weather resistance, low flammability thermal stability and outstanding electrical properties. Like other thermoplasts, FEP is easily molded to coated wires, tubes, pipes, foils and films.
Because it has excellent thermal stability and is practically non-flammable, FEP is frequently used for plenum constructions to meet fire resistance requirements. It is also the natural choice in data transmission cables due to its excellent dielectric properties. See EP 0 423 995 A1.
High processing speeds are desired when wires and cables are extrusion coated. Such high extrusion rates are limited by the occurrence of melt-fracture like with many thermoplasts. Melt-fracture results in surface roughness and/or uneven wall thickness. To increase the extension speed the molecular weight distribution of the used copolymer is believed to be very broad as disclosed, for example, in U.S. Pat. No. 4,552,925 for FEP.
For substantially broadening of the molecular weight distribution, the copolymer is mostly used as a mixture of at least two FEP's with largely differing molecular weights. The molecular weights are often characterized by the melt viscosity or the melt flow index (MFI-value). The desired mixtures are often produced by polymerizing the components separately and mixing them in form of the latices, reactor beads or fluff before melt pelletizing. Thus the manufacturing of these mixtures is a cumbersome and costly process.
Other FEP-mixtures are disclosed in DE 2,613,642 and DE 2,613,795.
These mixtures were claimed to be advantageous for diminishing the foaming during the stabilization process of FEP. This process is carried out by treating the resin at high temperatures up to 400° C. preferably in presence of water vapour. By this process the thermally unstable endgroups, mostly COOH and CONH2 groups (easily detected by IR-spectroscopy), are removed.
These mixtures have a very broad molecular weight distribution which according to conventional wisdom, results in an improved extrudability.
Removal of thermally unstable end-groups is required for the processing of FEP, in particular for wire-coatings. The decomposition reaction of the unstable endgroups, described in Modern Fluoropolymers, Ed. John Scheirs, Wiley & Sons 1997, p. 228 leads to bubbles and holes in the end-articles. Melt pelletizing of unstabilized polymer resins results in corrosion of the equipment used in the process and in metal contamination of the melt pellets. However, the stabilization process of DE 2,613,642 and DE 2,613,795 is very difficult to manage due to corrosion of the equipment because of the use of a water steam.
Metal contaminants are difficult to cope with. Such metal contaminants may result in degradation and decomposition of the copolymer at high processing temperatures. Decomposition generally leads to discoloration and degradation, and to a build up of die drools. Die drools are accumulations of molecular fractions of the polymer at the surface of the die exist. Die drools impair the coating processing. Also, cone breaks can occur. During the process of coating a wire, the molten polymer is extruded as a tube or sheath and drawn by vacuum onto the wire. Cone breaks are the discontinuities or breaks that occur during this process. Every time a cone break occurs, the coating process has to be re-initiated and one must wait for the system to reach equilibrium. Thus long processing times are more difficult to achieve. Productivity is diminished.
Furthermore, extrusion temperatures have to be kept as low as possible to counteract the decomposition reactions and resulting toxic off-gases, the rate of which substantially increases with elevated temperatures. On the other hand, lower extrusion temperatures result in higher melt-viscosities and thus an earlier onset of the melt fracture. Lowering the intrinsic melt viscosity by lowering the molecular weight results in poorer mechanical properties.
As a result, the materials should be made more thermally stable not only by eliminating the thermally unstable endgroups but also by avoiding metal contaminants and Mw fractions which are more prone to shear and/or thermal degradation.
Another way to eliminate unstable endgroups is postfluorination as disclosed in, for example, GB 1 210 794, U.S. Pat. No. 4,743,658 and EP 0 457 255 B1. Generally, elemental fluorine diluted with nitrogen is used at elevated temperatures up to close to the onset of melting of the polymer. The polymer may be subjected to fluorination in form of melt pellets, agglomerates or fluff. Here, too, excessive metal contamination should be avoided.
EP 0 222 945 B1 discloses the fluorination of hardened agglomerates, there called granules.
The fluorination leads to perfluorinated endgroups whereas the humid heat treatment as mentioned above, mechanistically cannot result in a fully fluorinated polymer resin. It is believed that inserted double bonds are present in the backbone leading to an inherent thermal unstability. These kinds of bonds may lead to a discoloration at long exposures to high temperatures.
There is another degradation reaction of FEP disclosed in U.S. Pat. No. 4,626,587. The onset of this reaction is supposed to occur by splitting the HFP diads in the middle of the chain at temperatures above the melting point. Such diads are formed at the radical polymerization by recombination of the correspondent polymer radicals as a termination step. The destruction of the diads at processing conditions leads to having the molecular weight of these polymer chains and hence to negatively affecting the mechanical properties, and to formation of more unstable endgroups. As U.S. Pat. No. 4,626,587 teaches, such diads are destroyed by subjecting the material to very high shear rates at temperatures far above the melting point. This process also is very costly.
There is another process disclosed in EP 0 789 038 A1 to reduce the backbone instability. The process is disclosed to use relatively large amounts of a chain transfer agent to suppress termination of polymer radicals by recombination.